Radio antenna for vehicle window

ABSTRACT

An antenna, particularly for use in VHF radio reception in a vehicle, is disclosed. The antenna is formed from an array of conductors which may constitute a window heating arrangement. The array has a receiving zone which is isolated from its surroundings by resonant isolating zones. These may be formed by interconnecting adjacent-heating elements which may be at a distance of 0.25λ of the wavelength (or multiple thereof) of the signal to be received from a real or virtual low-impedance connection to the vehicle structure.

The present invention relates to antennas. The antennas to which thisinvention relates will most typically find application in a vehicle andcan be used for VHF radio reception in the range of 76-110 MHz. However,antennas of the present invention may be used in other circumstances andother ranges (VHF otherwise) and are not restricted to use with audiobroadcasts.

A motor vehicle, being a cage of metal, is internally largely shieldedfrom external radio signals. It is thus necessary to provide an antennafor a radio receiver operating within the vehicle.

Traditionally, antennas for motor vehicles comprise a metal mast or rodwhich projects, in use, from the vehicle body. The disadvantages ofthese-have been long-recognised, such that technology has been availablefor many years whereby an antenna can be formed from conductive elementson a glass pane of the vehicle such as those used for rear-screenheating. Such antennas, in their broadest sense, will be referred toherein as window mounted antennas.

One reason why window mounted antennas are not universally used is thattheir cost is greater than the equivalent metal mast or rod antenna.This has not been due to the cost of providing a special glass pane;this is negligible. Rather, this has been due to the cost of theinterface circuitry required. Most particularly, the interface circuitryhas included active components for amplification of the signal receivedto a level suitable for feeding to a radio receiver.

An additional disadvantage of window mounted antennas (whichconventionally include active components) is that the signal-to-noiseratio of the output from such antennas has not been as good as that oftraditional mast types.

Various attempts have been made to improve the performance of windowmounted antennas. These have included variations in the interfacecircuitry, changes to the pattern of conductive elements, and providingseparate conductive elements dedicated to radio reception and which playno part in heating the window. However, these attempts have not removedthe above disadvantages.

The primary aim of the present invention is to provide a window mountedantenna, particularly but not exclusively for VHF reception in cars,which has a lower cost and better performance than has hitherto beenavailable.

In arriving at the present invention, the applicants have recognisedthat there has been acceptance that a window mounted antenna will bedisadvantaged through being mounted within a conductive surround ofuncontrolled behaviour at radio frequencies. A rear screen of a vehiclehas properties similar to a slot, in a ground plane, but its resonanceproperties are uncontrolled and correspond only by coincidence withfrequencies of signals to be received. Previously, attempts have beenmade to improve the signal derived from a disadvantageously disposedantenna. However, this has constituted an appeasement of the symptoms ofinherent deficiencies rather than any attempt to remove them.

By the present invention, there is provided an antenna for receivingradio signals in a vehicle within a desired frequency range comprisingan array of conductive elements disposed on a window pane, characterisedin that the elements are disposed to define a radio reception zonebetween boundary conductive paths, each boundary conductive path being apart of a respective loop tuned to resonate at a frequency within thedesired frequency range, the boundary conductive paths isolating thereception zone from the periphery of the pane to mitigate the effects onthe receptor zone of image current flowing in the periphery of theaperture and such that the antenna is favourably matched to radiosignals within the desired frequency range.

By realising that the antenna must be considered to act as a system inconjunction with its immediate surroundings, the applicants have beenable to provide an antenna which generates from the outset signals whichare of high quality. Particularly in the case of a metal vehicle theinteraction between the antenna and the surrounding vehicle body ishighly significant. In many embodiments, the signals can be ofsufficient magnitude to be useable by a radio receiver withoutamplification.

Preferably, the elements are disposed such that a reception zone iscreated in the array which at least partially compensates for theeffects of canceling image currents in the conductive material of thevehicle. In this manner, the reception zone can be configured asrequired to of-far high quality reception of signals. In suchembodiments, a connection may conveniently be made to one or moreelements within the reception zone from which connection an outputsignal is obtained.

In embodiments of the last-preceding paragraph, the reception zone maybe defined between a pair of boundary conductive paths, each of which ispart of a conductive loop tuned to resonate at a frequency within thedesired reception range, Each of such loops advantageously has anexternal connection (for example, to a surrounding vehicle body) of lowimpedance at a frequency within the desired reception range. Theexternal connection can, for example, comprise a series-resonantcircuit, or an open-circuit (1+2n)λ/4 transmission line, to implement ashort circuit at the frequency of operation. (Alternatively, a shortcircuit transmission time of nλ/2 may be used.) Such transmission lineshave the advantage that they can be formed as a conductive path on thewindow pane. In any case, the connection advantageously is of highimpedance to low frequencies and to DC.

Each boundary conductive path, as defined above, is preferably connectedto the respective connection to the vehicle body through multipleconductive paths, these multiple paths each being of lengthapproximately one quarter of the wavelength of a signal to be receivedwhile propagating within the window pane. These multiple paths areconveniently substantially parallel to one another and the boundaryconductive path comprises elements interconnecting adjacent ones of themultiple paths.

The multiple paths are typically formed by heating elements for thewindow pant. In such embodiments, the boundary conductive pathconveniently comprises a plurality of conductive elementsinterconnecting adjacent heating elements. In such embodiments, theinterconnecting elements are advantageously disposed such that theyinterconnect points of substantially equal potential of the electricalheating supply. In this way, substantially no heating current will flowthrough them, allowing them to be formed as fine conductors.

In an antenna of the present invention, typically all of the conductivepaths are formed by printing or deposition onto the pane.

In a second of its aspects, the invention provides a glass pane for avehicle comprising an array of conductive elements disposed toconstitute a heater for the pane and an antenna for receiving radiosignals of a desired range of frequencies; the array comprising aplurality of parallel heating elements extending between a pair of busbars, and a plurality of interconnecting elements each extending betweenadjacent heating elements, the interconnecting elements being disposedto be at a high impedance locus for signals in the desired range withrespect to a connection point on one of the busbars.

Such a glass pane may be fitted to a motor vehicle during manufacture toprovide that vehicle with an antenna for receiving radio broadcasts.

In a glass pane embodying the invention, the distance from a connectionpoint along the conductive path defined by the busbars and the heatingelements to each interconnecting element may typically be approximatelyone quarter of the wavelength of the signals of the desired frequencypropagating within the glass pane, although other distances may be usedand compensated for in the design. It is to be remembered that suchsignals will be propagating at a speed substantially less than (forexample 60% of) their speed in free space.

The array of conductors typically includes an output conductive elementconnected to an approximately central part of one or more of the heatingelements or two or more output elements that are later electronicallycombined. They may be symmetrically placed on the screen. A terminal maybe connected to the output conductive element or combined conductiveelements as the case may be, from which a signal is fed to a radioreceiver. Additionally, the array may include a conductive stripextending form each connection point adjacent one or more edges of thepane to act as a transmission line. Moreover, there may be a furtherconductive element to constitute a capacitive coupling member which maytypically comprise a T-shaped or L-shaped element connected to one ofthe heating elements, the crossbar of the T or L being disposed adjacentto an edge of the pane.

A glass pane embodying this aspect of the invention may comprise aconductive strip constituting a phase adjustment member operative toconcentrate the net signal currents in the centre of the screen.

The busbars of a glass pane as defined above are advantageously tuned toresonate within the desired frequency range.

In a third of its aspects, the invention provides a vehicleincorporating a pane of glass according to the second aspect of theinvention for use as a radio antenna.

In a fourth of its aspects, the invention provides an antenna forreceiving broadcast VHF radio signals in a vehicle the antennacomprising:

an array of conductive elements formed an a window pane of the vehicle,the array comprising first and second busbars extending close torespective opposite edges of the pane;

a plurality of generally parallel, spaced-apart heating elementsinterconnecting the busbars;

characterised by a respective connection of low impedance to thereceived radio signals from each busbar to electrically-conductivematerial of the vehicle surrounding the window pane;

by a plurality of interconnecting elements each interconnecting elementextending between adjacent heating elements, the interconnectingelements being approximately in two curved loci each disposed around arespective one of said connections of low impedance, and each locusbeing defined by the path length from the respective connection of lowimpedance, along the busbar to which that connection is made, and thencealong each heating element to the interconnecting element being adistance of λ/4+nλ/2) where n≧0 and λis the wavelength of a signal to bereceived while that signal is propagating within the window pane;

and by a T-shaped or L-shaped element connected to one of the heatingelements, she crossbar of the T or L being disposed adjacent an edge ofthe pane.

Embodiments of the invention will now be described in detail, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 shows a rear screen heater for a car incorporating an antennaembodying the invention;

FIGS. 2 and 3 are respectively first and second alternative arrangementsfor a low-impedance connection for earthing points in embodiments of theinvention;

FIG. 4 shows a rear screen for a vehicle being a second embodiment ofthe invention; and

FIG. 5 is a Smith chart of the performance of the antenna of FIG. 4.

With reference to FIG. 1, an embodiment of the invention comprises aglass rear screen 1 (known in the art as “a heated backlite”) for a caron which an array of conductive elements is formed in a mannerconventionally used to form a rear screen heater.

The array comprises a pair of busbars 10 which are generally paralleland spaced apart to be disposed adjacent to opposite edges of the screen1. The busbars 10 are interconnected by a multiplicity of heatingelements 14, these being generally parallel and meet the busbars at aregular spacing. A DC voltage derived from the electrical system of thevehicle can, by means of a user control, be selectively applied acrossthe busbars 10, this causing a heating current to flow in the heatingelements 14, with the effect of clearing frost or mist from the screen1. As thus far described, the array constitutes a conventional heatedscreen arrangement.

In accordance with the present invention, the structure also operates asan antenna for receiving radio transmissions within a desired frequencyrange, in this embodiment, the VHF range of 67-110 MHz.

Each busbar 10 is connected at a respective point A to the vehicle bodythrough a path of low impedance to signals within the desired frequencyrange. With this embodiment, such connection is made through aseries-resonant circuit 16, comprising a series-connected capacitor andinductor, to the vehicle body at 18. The series-resonant circuit istuned to resonate within the desired frequency range, such that theseries-resonant circuit 16 provides a low-impedance path to the vehiclebody for signals of such frequencies, but is effectively open-circuitfor DC signals.

A series of interconnecting conductive elements 20 are provided whichinterconnect adjacent heating elements 14. The interconnecting elements20 are disposed such that they interconnect points on the heatingelements which are of a distance traced along a conductive path oftypically 0.25λ from the point A of a busbar 10. Where a low-impedanceconnection at the frequency of operation is implemented to the apertureperiphery, this is typically the point at which DC power is supplied tothe heater, and symmetrically the point at the DC path to the vehicleearth. As used herein, λ is the wavelength of signals to be received asthey propagate in the glass pane. (It is to be remembered that radiosignals propagate in conductive tracks printed on glass by a typicalfactor of 0.6 of their speed in free space, their wavelength beingshortened accordingly.) Thus, as shown in FIG. 1, the interconnectingelements 20 are disposed on two loci, each centred on a respective pointA.

The interconnecting elements 20 are disposed transversely to the heatingelements 14 so as to interconnect points of substantially equal DCpotential arising from the heating current. In this way substantially nocurrent flows through them, so minimising their interference with theheating effect of consequent interference with vision) to be minimised.

Each point A has associated with it a respective plurality ofinterconnecting elements 20, This divides the entire array into threeregions, the centre of which 22 constitutes a receiving zone for signalsof the desired frequency. The closed loop provided from each point A,through the interconnecting elements is a half-wave resonant structure.It has been found that the structure of the outer zones 24 serves toisolate the receiving zone 22 from the effects of the surroundingvehicle, allowing it to operate substantially as a slot antenna.

An output conductive element 26 is connected to a centre point on two ofthe lowermost heating elements 14. The output element 26 is connected toa suitable terminal at which connection is made to a co-axial feed wire28 to carry a received radio signal to a radio receiver.

With reference to FIG. 2, a first alternative to the series-resonantcircuit described above is shown, this having the advantage of needingno discrete components. In this arrangement, a conductive stripconnected to the vehicle body 30 is provided surrounding the screen. Aseries resonant circuit is a constituted by a resonant conductiveelement formed as part of the array connected to the busbar at a pointA. The resonant strip comprises a first region 32 which is convoluted toform an inductor, and a second T-shaped capacitive region 34 lyingadjacent the earthed strip 30, to be capacitively coupled therewith.

In this embodiment, the earthed strip 30 is not strictly necessary, itbeing possible to capacitively couple directly with the vehicle bodyinstead. However, it has been found that this is difficult to control,particularly where a screen is secured to the vehicle by means ofadhesive, the presence of adhesive between the capacitive region 34 andthe vehicle body substantially increasing the effective resistive lossassociated with the reactance of the capacitance.

A further alternative to the series-resonant circuit described withreference to FIG. 1 is shown in FIG. 3, which is potentially morespace-efficient than the embodiment described in the last-precedingparagraph. In this embodiment, there is provided connected to anearthing point A, a conductive element 40 which constitutes atransmission line. This is disposed to extend for a length of 0.25λ, or0.25 (1+2n)λwhere n is a positive integer, adjacent the vehicle body ora peripheral strip 30, as described above. This arrangement constitutesa tuned stub which is effectively a short circuit for those signals towhich it is tuned.

With reference to FIG. 4, there are various enhancement a which can anbe made to optimise the performance of antennas embodying the invention,For example, it is desirable that the output from the antenna as closelyas possible matches the input impedance of the radio receiver, typically120Ω.

Firstly, it may be desirable to tune the busbars 10 to resonate close tothe centre of the desires range of frequencies. This can be achieved byextending them as shown at 50.

Optimisation can be further enhanced by providing capacitive couplingelements, such as those shown at 52, to couple the receiving zone 22 toits surroundings. Furthermore, elements ouch as those shown at 54 can beprovided to adjust the phase of the signals within the receiving zoneand so reduce losses due to circulating currents which may occur in thelower heating elements which are interconnected by the output element26.

The Smith chart of FIG. 5 shows the high standard of performanceachievable with this embodiment normalised to 120Ω.

It will be appreciated that many variations are possible within thescope of the invention, as defined in the following claims. For example,it is possible for a signal to be taken from the receiving zone 22through one or more additional connections, or by inductive orcapacitive couplings, Diversity reception is possible using embodimentsof the present invention in true orthogonal modes of resonance.

Additionally, it will be appreciated that the physical point at whichconnection is made to the busbar 10 may not coincide with the point A.By use of suitable networks, these may be moved from point A while stillretaining a low-impedance coupling at the aperture edge at this pointthe coupling being an image of the complex impedance presented at theconnection point.

What is claimed is:
 1. An antenna for receiving radio signals in avehicle within a desired frequency range comprising an array ofconductive elements disposed on a window pane, wherein the elements aredisposed to define a radio reception zone between boundary conductivepaths, each boundary conductive path being a part of a respective loopturned to resonate at a frequency within the desired frequency range,the boundary conductive paths isolating the reception zone from theperiphery of the pane to mitigate effects on the reception zone of imagecurrent flowing in the periphery of a window aperture.
 2. An antennaaccording to claim 1, wherein each said loop has an external connectionof low impedance at a frequency within the desired frequency range tothe vehicle body at the window aperture.
 3. An antenna according toclaim 2, wherein each said connection to the vehicle body comprises aseries resonant circuit.
 4. An antenna according to claim 3, whereineach said connection to the vehicle body comprises a transmission heconfigured to produce a short circuit across the connection at afrequency of signals to be received.
 5. An antenna according to claim 4,wherein said transmission fine comprises a conductive path disposed onthe window pane adjacent the edge of the window pane.
 6. An antennaaccording to claim 2, wherein the connection to the vehicle body has ahigh impedance to low-frequency signals.
 7. An antenna according toclaim 2, wherein each boundary conductive path is connected to therespective connection to the vehicle body through multiple conductivepaths, these multiple paths each being of length λ/4+n(λ/2) where n≧0and λ is the wavelength of a signal to be received while that signal ispropagating within the window pane.
 8. An antenna according to claim 2,wherein each boundary conductive path is connected to the vehicle bodythrough multiple conductive paths, these multiple conductive paths beingof such a length as to produce an image of a high-impedance locusequivalent to λ/4 from the respective low impedance connection pointwhere λ is the wavelength of a signal to be received while that signalis propagating within the window pane.
 9. An antenna according to claim2, wherein each boundary conductive path defines a locus of equaldistance for a signal propagating within the window pane from the lowimpedance points on the respective sides of the heated rear window of avehicle.
 10. An antenna according to claim 9, wherein the multiple pathsare substantially parallel to one another and the boundary conductivepath comprises elements interconnecting adjacent ones of the multiplepaths.
 11. An antenna according to claim 10, wherein the boundaryconductive path comprises a plurality of conductive elementsinterconnecting adjacent heating elements.
 12. An antenna according toclaim 11, wherein the boundary conductive path comprises a plurality ofconductive elements interconnecting adjacent heating elements.
 13. Anantenna according to claim 12, wherein the interconnecting elements aredisposed such that they interconnect points of substantially equalpotential of the electrical heating supply.
 14. An antenna according toclaim 1, adapted to receive VHF radio signals.
 15. An antenna accordingto claim 1, wherein said matching is effective over a bandwidth offrequencies to be received.
 16. An antenna according to claim 1, whereina connection is made to two elements within the reception zone fromwhich connection an output signal is obtained.
 17. An antenna accordingto claim 11, wherein all of the conductive paths are formed by printingor deposition onto the pane.
 18. An antenna according to claim 1,wherein the pane is a window mounted in an aperture of a vehicle body.19. An antenna according to claim 18, wherein the array of conductiveelements includes tuning element arranged to fink the array capacitivelyto the vehicle body by means of which the resonance of the array andvehicle body system can be tailored to suit the signals to be received.20. A glass pane for a vehicle comprising an array of conductiveelements forming both a heater for the pane and an antenna for receivingradio signals of a desired range of frequencies, said array comprising aplurality of parallel heating elements extending between a pair ofbusbars, and a plurality of interconnecting elements each extendingbetween adjacent heating elements, the interconnecting elements beingdisposed at high impedance, loci for signals in the desired range withrespect to a connection point on one of the busbars.
 21. A glass paneaccording to claim 20, wherein the distance from a connection pointalong the conductive path defined by the busbars and the heatingelements to each interconnecting element is approximately an oddmultiple of one quarter of the wavelength of the signals of the desiredfrequency propagating within the glass pane.
 22. A glass pane accordingto claim 20 wherein the array includes an output conductive elementconnected to an approximately central part of at least one of theheating elements, a terminal being connected to the output conductiveelement from which a signal is fed to a radio receiver.
 23. A glass paneaccording to claim 20, wherein me array includes a output conductivestrip extending from each connection point adjacent at least one edge ofthe pane to act as a transmission line.
 24. A glass pane according to 20further comprising a conductive element arranged to constitute acapacitive coupling, member.
 25. A glass pane according to claim 24,wherein the capacitive coupling member comprises a T-shaped or L-shapedelement connected to one or more of the heating elements, the crossbarof the T or L being disposed adjacent an edge of the pane.
 26. A glasspane according to claim 20, further comprising a conductive stripconstituting a phase adjustment member operative to optimize signalcurrents in the center portion of the screen thus minimizing the effectof deleterious image currents in the material of the vehicle body.
 27. Aglass pane according to claim 20, wherein the busbars are tuned toresonate within said frequency range.
 28. A glass pane according toclaim 27, wherein each of the bus bars has an associated plurality ofinterconnecting elements.
 29. A vehicle incorporating a pane of glassaccording to claim
 20. 30. An antenna for receiving broadcast VHF radiosignals in a vehicle, said antenna comprising: an array of conductiveelements formed on a window pane of the vehicle, the array comprisingfirst and second busbars, extending close to respective opposite edgesof the pane; a plurality of general parallel, spaced-apart heatingelements interconnecting the busbars; a respective connection of lowimpedance to the received radio signals from each busbar toelectrically-conductive material of the vehicle surrounding the windowpane; pane; a plurality of interconnecting elements each interconnectingelement extending between adjacent heating elements, the interconnectingelements being approximately in two curved loci each disposed around arespective connection of low impedance, and each locus being defined bythe path length from the respective connection of low impedance, alongthe busbar to which that connection is made, and thence along eachheating elements to the interconnecting element being a distance ofλ/4+n(λ/2) where n÷0 and λ is the wavelength of a signal to be receivedwhile that signal is propagating within the window pane; and a T-shapedor L-shaped element connected to one of the heating elements, thecrossbar of the T or L being disposed adjacent an edge of the pane.